damage control

[boog]

last spring i built a TS clone on one of the GGG boards. used a 2272 chip. sounded nice. gave it to a friend. he liked it. & he nuked it. so i'm looking fr some advice on repair and here's what i've seen:

the chip is split in half.
the socket is melted around pins 5 & 6.
the 100uf power cap (rated 16v) looks as though it's about to burst.

i'm guessing he plugged a 18v or 24v adapter into the thing (not gonna get all 'battery vs adapter' here, but i don't install batteries in mine). so, obviously, the chip is toast as well as the power cap. should i replace the diode as well? after that i'll just try standard debug to figure out nuked parts, but after all this time i still don't necessarily understand electronic theory that well. as always, yr advice is appreciatively received.

[Earthscum]

replace the chip, cap, and use a meter to test the diodes. Should be ok, but if you have any problems, check those.

I've split resistors before and didn't know until I checked for continuity (after replacing a bunch of other stuff).

Find a nice spot on the trace side of the board, or across your adapter jack, and drop in a 12V zener for protection. One direction you're limited to 12V, and if your buddy grabs a reversed polarity adapter, it will only send about -0.6V to the circuit components, saving them.

[boog]

check. thanks much!

the 12v zener, it would run parallel with the zapped cap? from (+) to ground, correct? could i replace the 1n914 with said zener? or would the two components be serving different purposes?

while we're here, using the zener in this fashion, if i'm correct, would serve much the same purpose as a 7805? where the 7805 regulates the voltage to 5v, the zener limits the voltage to 12? not exactly the same, i understand, but similar?

[greaser_au]

while we're here, using the zener in this fashion, if i'm correct, would serve much the same purpose as a 7805? where the 7805 regulates the voltage to 5v, the zener limits the voltage to 12? not exactly the same, i understand, but similar?

they're kinda opposite ideas, the 78XX is like a (series) tap from the supply, it turns on enough to maintain the output voltage at a high enough level.  A zener is like a (shunt) tap to the drain, you turn it on  more to drop the level if it gets too high...

However, it's really a little more complicated than that - you need a (carefully calculated) resistor to use a zener as a regulator. The application here is without a resistor (yes, supply series resistance will limit current) so it's a  'hard clipper' to protect the circuit from overvoltage (but if your mate is really clever he will test your OV protection to destruction ) I'd use as big zener as you can find (that will fit) - say a 1W.  

edit: yes, if that 1N914 is directly across the supply, a 12v zener can replace it in this specific situation.

david

[boog]

excellent david! it had never occurred to me that th 78XX could bring voltage up! just as soon as i think i've gotten something figured out, i learn something else. so thanks again for giving an illustration i can understand 

[R.G.]

i'm guessing he plugged a 18v or 24v adapter into the thing (not gonna get all 'battery vs adapter' here, but i don't install batteries in mine).

It may well be that he plugged in a 9V **AC** adapter. I've seen them do things like this, even to pedals that were nominally polarity protected.

so, obviously, the chip is toast as well as the power cap. should i replace the diode as well?

Replace everything you have even a whiff of concern about. It might be simpler to simply clip off all the parts and replace them all. With so many things already damaged it's not simple to say that any one part is not pushed to the edge of failure. Parts on boards are cheap. Field failure is not.

excellent david! it had never occurred to me that th 78XX could bring voltage up!

It can't. 78xx regulators can never produce output voltages larger than 2V down from the input voltage, or the regulated voltage if the input voltage is more than 2V larger than the regulated voltage.

The real screaming advantage of series regulators like the 7800 series is that they only let through enough current to supply the load. Shunt regulators like zeners have to be designed to let through the maximum design current all the time and the zener "eats" any excess that the load doesn't need.

[boog]

as always, r.g., thank you for clarifying my misunderstanding. i wonder, is there a particular article you may know of that i could go to for further understanding?

i'm going to defer to the guitarist as to how far he wants me to go in replacing parts; swapped the power cap, chip & put the 12v zener in for the 914 and she works splendidly again. just have to give the string plucker a stern talking to about power supplies 

[andymac1962]

bulging capacitor can either be too high a voltage applied, reverse voltage, or an AC voltage applied
TLC2272 & TLC2262 are only rated for 16V supply absolute maximum, so if a 18V DC or higher supply was used, the chip is dead.

Replace the capacitor, ic & diodes as appropriate, and make sure your friend know what DC supply to use, or better still disable it, & force him to use a battery.

[Hides-His-Eyes]

or better still disable it, & force him to use a battery.

Slow down there grandpa!

The parts to make a pedal run on literally any power supply made with a barrel jack up to 18V, maybe even more, would cost you less than a single battery, and wouldn't have any of the environmental impact . They also take up less room. And don't run out on stage

[greaser_au]

excellent david! it had never occurred to me that th 78XX could bring voltage up!

It can't. 78xx regulators can never produce output voltages larger than 2V down from the input voltage, or the regulated voltage if the input voltage is more than 2V larger than the regulated voltage.

I stand corrected.. what I should have said is: the 78XX behaves like a tap from a *higher supply voltage*...  


david

[Paul Marossy]

So the IC was literally split in half?! Whoa. 

[boog]

yep, not clean in half like it came out in 2 pieces, but theres a crack running the length of it equidistant from each set of pins that seems to go most of the way through the body.  if i had a camera i'd post a pic for you.  i am keeping it as a sort of 'trophy' 

one of the reasons i don't use batteries in my pedals is that i'm not really good at leaving space for one. having nuked a few builds in the past i did switch to buying components (namely caps) rated 35v and up but thank you for reminding me to check the data on my chips.

i did pull a switcheroo on him and we'll see what he thinks: i put the 2272 in originally because it definitely ran 'hotter' in terms of the way the guitar sounded compared to the 4558's that i had; this time i put in a 4558 that was salvage from some old powered computer speakers. so while it may not have the mojo of the old 4558's, it does have a bit of voodoo in using one dead thing to bring another to life     franken-screamer!

[Paul Marossy]

I've never heard of an opamp doing that before. It seems to me like they tried powering it with an AC adapter or something. Or with a wall wart that supplied way too high of a voltage. 

[boog]

dunno. he's an acoustic guy going electric & pretty much has no idea what's going on between guitar and amp. much like i was when i started doing this, each pedal might as well be powered by magical unicorn tears.

[Paul Marossy]

dunno. he's an acoustic guy going electric & pretty much has no idea what's going on between guitar and amp. much like i was when i started doing this, each pedal might as well be powered by magical unicorn tears.

Ooo... good one for marketing hype!

"powered by magical unicorn tears"